This invention relates to an eye refractometer for automatically measuring the refractive power of the eye.
As an example of the abovementioned eye refractometer, there is a known apparatus comprising a measuring target projection system including two light sources arranged at positions symmetric about an optical axis and a target moving on the optical axis and two light receiving elements arranged at positions conjugate with each other with respect to an objective lens and symmetric with respect to the optical axis. In this apparatus, the beams of light emitted from the two light sources, which are alternately lit, passes through the target at a certain angle relative to the optical axis and are projected onto the eye fundus. The refraction is calculated in terms of a quantity of movement of the target when the target is moved until the two target images of both beams of light on the eye fundus are formed at the same position and the amounts of light received by the two light receiving elements become equal to each other.
However, the conventional eye refractometer has the following two drawbacks. The first drawback is that the apparatus measures refraction in only the meridian direction including the optical axis and the two light sources. In order to measure the refraction in an other meridian direction, the two light sources as well as the two light receiving elements must be rotated through the same angle about the optical axis as the center. However, the construction of such an apparatus becomes complicated and it fails to provide a high level of accuracy because a large number of moving parts are necessary. In another type of an apparatus for measuring the refraction in a plurality of meridian directions, an image rotator is disposed inside a common optical path of the measuring target projection system including the two light sources and the target and a detecting optical system including the light receiving elements. When the image rotator is rotated, the same effect can be obtained as when both the measuring target projection system and detecting optical system are simultaneously rotated in the same direction by the same angle. In this kind of the apparatus, however, the image rotator becomes large and the optical length of each of the measuring target projection system and the detecting optical system must be elongated in order to permit the image rotator to be inserted, inviting difficulty in design. Moreover, a ghost flare is likely to occur because the image rotator includes a large number of reflecting surfaces.
The second drawback is that the measurement is very time consuming because the target must be moved on the optical axis to focus the target image on the eye fundus whenever the refraction is measured in each radial direction. This results not only in the fatigue of the patient but also in changes in the measuring conditions of the eye to be inspected during measurement so that measurement can not be done with a high level of accuracy.
As still another example of a conventional eye refractometer, an apparatus is also known which comprises the measuring target projection system including the two light sources arranged at positions symmetric about an optical axis and the target fixed on the optical axis, a target image measuring system for measuring the distance between two target images on the eye fundus and an operation system for calculating the refraction from the distance measured by the target image measuring system. This apparatus has the problem that the measuring accuracy drops if the distance between the two target images becomes great, that is to say, the problem that the measuring range of the refraction becomes narrow.